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I refined my callibration steps using my CDK17 which is sensitive to good flat fielding. A slightly off master dark, or a weak flat or off temperatures or no bias subtract to the flat master at callibration would wreak havoc.
I do an average combine of flats - no bias subtract, to form a flat master.
Ray did an article about the number of flats to use for an ideal flat on this site a few months back.
I make a master bias and have that subtracted when applying the flats. This seemed to give a better result.
Doing dark subtraction seems to make images noisier with the Trius. The bias subtract though gets rid of a white line along the left side of the image. This does not seem to show in final images if not removed but probably better if it is. So I have also started doing a bias subtract as well as a flat if the image has dust donuts. If no dust donuts (better to clean filters carefully with a photographic cleaning cloth and a blower) then no flats may give the better image as it introduces some noise.
There is a paper by Richard Crisp about flat ADU levels. He advises the opposite here of doing higher ADU values like 30-40K. I am sure its on his website if you look for it. I tend to go for between 20-30K but in the end I check to see what works and adjust. Sometimes I went for lower ADUs with the CDK17 like 20K as 30K sometimes overcorrected and made the corners too bright.
Hot/cold pixel removal during data rejection seems to clean up anything remaining. I used to see the odd coloured dots in final images before I used that now I rarely do.
As the subs tend to be shorter with this camera lots of subs tends to make these statistical methods work better as they all work by identifying what is an outlier (outside the "norm") one way or the other.
I find median combine works best for combining images. I believe bi cubic sampling is a bit old hat and not best (I could be wrong). Lanzcos 36 is better but its slow. How much difference these little points make is debatable to the final image quality but I suppose every little bit helps.
I find my resulting master Trius images are nice and clean this way.
If I do use flats I may be inclined to use a master dark. I find though these tiny Sony chips tend not vignette to any real degree with the scopes I have been using.
Sony chipped cameras require a different approach than Kodak based cameras which are way noisier in comparison with usually 3 to 4 times the read noise and 2/3rds or less the QE with way weaker QE in Ha and O111, S11. If Sony could only make a full frame ExHad 11 CCD nobody would use the Kodaks.
A defect map would be an interesting alternative to a dark subtract. If you do make one I'd like to hear of your results. I find though the very few hot pixels are gone by the time I do the above.
Greg.
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